Cache ensemble contour map statistics as GRI surface files

[magnesj]

Persist computed ensemble contour map statistics to a project-adjacent cache, so that reloading a project skips the sweep over the ensemble realizations entirely. Stacked on #14408, which computes all pending sibling contour maps in one sweep; together they address the slow project load described in #14319.

The cache is written in setupBeforeSave() following the RimReservoirCellResultsStorage lifecycle pattern: one GRI (binary IRAP) surface file per statistics type per selected time step in a <project>_cache directory, written and read with the existing RifSurfio. The surface nodes are placed at the contour map cell centers, so every cached statistics map can be imported directly as a regular surface in ResInsight, xtgeo or RMS. Undefined cells round-trip as NaN. On load, ensureResultsComputed() restores the contour map grid and results from the cache when a validity key (MD5 of all settings affecting the result: result definition, aggregation, flooding settings, resolution, bounding box expansion, time steps, formations, polygon geometry and realization file paths) matches the current settings; the key is captured at compute time, so results computed under outdated settings are never cached, and any setting change falls through to recomputation. Malformed or missing cache files fall back to recomputation.

Cache metadata (validity key, grid definition, time steps) is stored in hidden PDM fields. An alternative design storing the metadata in a text footer inside the GRI files was implemented and reverted (last two commits): ResInsight's surfio importer ignores trailing bytes, but xtgeo reads value blocks to end-of-file and fails on such files, which would defeat the interoperability that motivated the GRI format. The cache file name is stored as a bare file name in a plain QString field, since caf::FilePath fields are rewritten by the global path list macros on save.

The branch also adds a binary ROFF writer (RifRoffWriter) with round-trip unit tests, developed for an earlier iteration of the cache format and kept as general infrastructure; it can be dropped from the PR if not wanted.

Verified with an ensemble test project: save writes the cache files, reload restores the maps with exports identical to a fresh computation, corrupt files and setting changes trigger recomputation, and the cache files import as surfaces in both ResInsight and xtgeo.

Fixes #14418